Our overall objective is to obtain a better understanding of the mechanism(s) involved in altered mitochondrial structure and function in aging by probing the role of protease(s) recently described in purified mitochondrial preparations (23,58) in these alterations. We have recently observed in our laboratory (A) increased mitochondrial neutral protease(s) activity in Aged Fisher 344 rats when compared with adult controls and suggest that these protease(s) could be involved in mechanism(s) underlying subtle alterations in mitochondrial structure and function which have been reported by a number of laboratories. Our observation opens a new horizon for the study of the mechanism(s) involved in altered mitochondrial structure and function and more importantly enable us to better evaluate the role of these changes in the biology of aging. Our immediate goals are: 1) to characterize the mitochondrial protease(s) in adult and aged animals and determine whether the increased mitochondrial protease(s) activity we have observed in aged animals is involved in loss of mitochondrial function and changes (including turnover and synthesis) in specific membrane components; 2) to extend our initial observations of increased CTP: phosphatidic acid cytidyl transferase activity and phospholipid/protein ratio in mitochondria of aged Fisher 344 rats by determining CTP: phosphatidic acid cytidyl transferase activity and cardiolipin synthesis in other aging animals as well as in aged animals in which in vivo cell renewal has been induced by partial hepatectomy; 3) to utilize the technology we have developed to continue our studies of mitochondrial three-dimensional structure in situ (in intact tissue and in isolated hepatocytes) as a consequence of aging. Our primary model will be Fisher 344 rats from the aging colony of NIA. For comparative purposes, other animals will be evaluated i.e., the Sprague-Dawley rat (continues to grow beyond sexual maturity), and partially hepatectomized rats (to compare mitochondria from newly created liver cells in old animals with those of old cells).